High strength cellulosic filament its use, and method for the production thereof

ABSTRACT

The present invention relates to high-strength, possibly pigment-containing—for example flame-retardant—regenerated cellulosic filaments having improved textile properties, in particular as regards strength and uniformity, to their use for the production of fabrics, and to a method for the production of these filaments.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to high-strength regenerated cellulosicfilaments having improved textile properties, in particular as regardsstrength and uniformity, to their use for the production of fabrics, aswell as to a method for the production of these filaments. Thesehigh-strength filaments may also contain pigments such asflame-retardant pigments.

Regenerated cellulosic filaments have been known for a long time. Theyare used particularly for textiles, but in a high-strength form also fortechnical applications such as tire cords. For the purposes of thisinvention, the term “regenerated cellulosic fibers” denotes such fibersthat are made of a cellulose-containing spinning solution by spinninginto a spinning bath (often also referred to as a precipitation bath),the cellulose in the spinning solution being present in the form of acellulose derivative, in particular as cellulose xanthogenate, while inthe precipitation bath a regeneration back into pure cellulose takesplace. The regenerated fibers and filaments produced via thexanthogenate are usually referred to as “viscose” or “rayon”. Also thecellulose-containing spinning solution is referred to as “viscose” oralso as “viscose spinning solution”. Therefore, the manufacturing methoditself is referred to as viscose method.

In principle, viscose processes for staple fibers and continuousfilaments have been known for many years and are, for instance,described in detail in K. Götze, Chemiefasern nach dem Viskoseverfahren,1967. However, the textile properties of the fibers and filamentsobtained thereby are influenced significantly by many parameters. Inaddition, the size of the existing production facilities imposes limitson many influencing variables, which cannot be exceeded for technical oreconomic reasons, so that arbitrary variations of the parameters areoften not even possible and would therefore not occur to those skilledin the art.

In the course of time, different variants of the viscose method weredeveloped and, to some extent, have found industrial use to the presentday. The different variants differ mainly in the composition of thespinning solution and of the precipitation bath, which may stronglyinfluence the mechanical properties of the products. Examples thereofare the Modal and the Polynosic fibers which, however, on a commercialscale, are only manufactured in the form of staple fibers. Thefundamentals of the Modal method for the production of staple fibers aredescribed in AT 287905. With respect to filaments, it has meanwhilebecome possible, through technical improvements of machinery, to spincontinuously by means of the so-called Continue method, instead of theformer discontinuous centrifugal methods.

Today, standard viscose filaments are widely used in the textile andclothing industries, particularly for lining materials. Nevertheless,the usability of viscose filaments for textiles is limited by their lowstrength, especially in the wet state, by their high elongation, and bytheir high area shrinkage. More specifically, they do not permit theproduction of lightweight, i.e., thin, yet strong textiles that can alsobe washed without problems. In addition, they are not suited for moreheavily stressed textiles, e.g. work clothing and uniforms, either.While, in this case, there exist some possible solutions with syntheticfilaments, for example filaments made of polyester or polyamide, theseare, especially when it comes to wearing comfort, significantly inferiorto cellulosic materials. Therefore, there continues to exist a need forhigh-strength cellulosic filaments for textile applications, whichcannot be produced cost-effectively using the known methods.

A known, commercially available, regenerated high-strength filament isCORDENKA®, for example, which is produced by a modified viscose method.It is produced with single-fiber titers of approx. 1.8 dtex andstrengths (conditioned) of approx. 45 to approx. 52 cN/tex. It is usedin the technical field to reinforce rubber goods, particularlyhigh-quality vehicle tires. For textile applications, such a filament istoo coarse and also too expensive to produce. In this case, especiallythe compositions of the spinning solution and of the precipitation bathwere changed, as compared to standard viscose.

Generally, it has also been known for a long time to spin variouspigments into regenerated cellulosic fibers, especially into fibersobtained using the viscose method. In that case, the pigments may beflame-retardant pigments, especially phosphorus-based pigments, or alsocolorants and matting agents, respectively. Such pigment-containingviscose fibers are produced worldwide with a single-fiber titer between0.8 and 16 dtex for standard applications in the textile and nonwovenssectors. In this context, the pigments are added to thecellulose-containing spinning solution using suitable dosing devices.Then, this pigment-containing spinning solution is extruded throughspinnerets and precipitated and subjected to further treatment accordingto the known methods. The pigments introduced into the fibers in thisway are very firmly embedded and can for example not be washed out usingconventional washing processes.

The relevant literature describes various chemicals that can be used toprovide viscose fibers with a flame-retardant finish. It is mainly flameretardants based on halogens, silicon, and phosphorus that are used forthis purpose.

Patents DE4128638A1 or DE102004059221A1 describe flame retardantdispersions based on a2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide, usingvarious dispersing agent systems, and they also mention the use of thesedispersions for providing viscose fibers with a flame-retardant finish.

EP1882760 also describes the production of flame-retardant viscosefibers by using flame retardant dispersions based on a2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide. It isdescribed therein that an important feature of the invention is that theparticle size must not be greater than 10 μm and that, prior tospinning, the spinning dope must therefore be cleaned using filtershaving a maximum mesh width of 10 μm. However, it has been found thatthis criterion is not enough to manufacture fibers that satisfy therequirements described herein. The maximum particle size of 10 μmdescribed in EP1882760 may perhaps be sufficient for continuous viscosefilaments or monofilaments having a large titer, however, it does notnearly satisfy the demands of a modern staple fiber production involvingfiber fineness levels of approx. 1 to 4 dtex; a 1.3 dtex fiber has adiameter of approx. 10 μm.

The fibers that are described in prior art documents or are commerciallyavailable are all produced by means of standard viscose methods. Infact, they feature comparatively good mechanical fiber properties forflame-retardant viscose fibers, as the phosphorus contents are very low.However, studies with various phosphorus-based flame retardants haveshown that a sufficient flame-retardant activity is only achievedstarting from phosphorus contents greater than 2.8%. In this case, theflame-retardant capacity correlates well with the content of retardantagent, converted into pure phosphorus.

It was found, however, that for example the incorporation of largequantities (15-25%) of a flame-retardant pigment caused a furtherdeterioration of the textile parameters of the viscose fiber. Therefore,the limitations of usability already mentioned in connection withstandard viscose fibers apply even more so to flame-retardant viscosefibers.

This is all the more regrettable, as flame-retardant fibers could beused to particular advantage in products that are also exposed to highmechanical stresses, for example, in the work clothing used forparticularly dangerous activities such as in fire brigades, foundries,in the military, and in the petroleum and chemical industries. For suchproducts, usually synthetic high-performance fibers such as (aromatic)polyamides, aramides, polyimides, and the like are already beingemployed. These fibers, however, offer a low wearing comfort, as theyare unable to absorb moisture to a sufficient extent. It would thereforebe desirable to have a mixture of these fibers with cellulosic fibersthat add enhanced wearing comfort to the spectrum of properties withoutcausing the remaining properties to deteriorate significantly.

It has also been known to spin color pigments or matting agents,particularly titanium oxide, into regenerated cellulosic fibers. In thiscase, the same problems will generally emerge because of the solidscontent. The problems described here caused by spinning in of largerquantities of solids apply to viscose staple fibers, however, even moreso to viscose filaments.

WO 2011/026159 A1 discloses a method for the production offlame-retardant cellulosic staple fibers, which is supposed to solvethese problems. The fiber described therein contains a spun-inparticulate phosphorus compound serving as a flame-retardant substance,preferably an organophosphorus compound, and has a so-called use valuebetween 6 and 35, preferably between 8 and 35, and more preferablybetween 10 and 35. Such a fiber was produced for the first time by meansof a modified viscose process.

For the production of the fibers described in WO 2011/026159 A1, acellulose concentration of 4-7%, when using a pulp having an R-18content of 93-98%, and an alkali ratio (=cellulose concentration/sodiumhydroxide concentration, each in g/l) from 0.7 to 1.5 have been found toconstitute the ideal conditions. However, because of the addition of theflame-retardant FR pigment, the spinning parameters must be adaptedaccordingly.

Therefore, WO 2011/026159 A1 also describes a method for the productionof a flame-retardant regenerated cellulose fiber for textileapplications by spinning a viscose containing 4 to 7% cellulose, 5 to10% NaOH, 36 to 42% (related to cellulose) carbon disulfide as well as 1to 5% (related to cellulose) of a modifier into a precipitation bath,withdrawing the coagulated threads, a viscose being used whose spinninggamma value is 50 to 68, preferably 55 to 58, and whose spinningviscosity is 50 to 120 falling ball seconds; and that the temperature ofthe precipitation bath is 34 to 48° C., where

a. the alkali ratio (=cellulose concentration/alkali content) of theready-to-spin viscose is 0.7 to 1.5;

b. the following precipitation bath concentrations are used:

-   -   H₂SO₄ 68-90 g/1    -   Na₂SO₄ 90-160 g/1    -   ZnSO₄ 30-65 g/1

c. the final withdrawal from the precipitation bath takes place at avelocity between 15 and 60 m/min; and

d. a pigment-type organophosphorus compound in the form of a pigmentdispersion is spun in as a flame-retardant substance.

Conveniently, a viscose is used to which the modifier has been addedonly shortly before the viscose is spun.

Together, the measures proposed in WO 2011/026159 A1, that is, to complywith a certain spinning ripeness represented by the spinning gammavalue, to comply with a certain viscosity represented by the fallingball values, and to comply with certain conditions in the precipitationbath, bring about the envisaged fiber properties. The spinning gammavalue denotes the proportion of carbon disulfide molecules bonded to 100cellulose molecules. The spinning gamma value is determined according tothe Zellcheming Draft Leaflet by R. Stahn [1958] and to Leaflet III/F 2,respectively. The falling ball is used in the falling ball method todetermine viscosity; it is expressed in falling ball seconds. Thedetermination is described in K. Götze, Chemiefasern [1951], p. 175.

The flame-retardant phosphorus compound that is produced as a pigmentis, according to WO 2011/026159 A1, added to the viscose spinningsolution in the form of a pigment dispersion. In this case, so much ofthe flame-retardant substance is spun in that the finished fibercontains at least 2.6%, preferably between 3.2% and 6.0%, morepreferably between 3.5% and 6.0% phosphorus, related to cellulose.

As has already been explained hereinabove, a flame-retardantorganophosphorus compound that is particularly well suited for thepurposes of the present invention is2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide.

The quality of the pigment dispersion, in particular, also has asignificant influence on fiber properties. This quality depends on theaverage and maximum particle sizes of the pigments, on the concentrationof the dispersion in use, i.e., when adding it to the viscose spinningsolution, as well as on the type and the quantity of the dispersingagents.

Contrary to the possible upper particle size of 10 μm described in thepatent EP1882760, it has been found that an average particle size (x₅₀)of less than 1 μm and a maximum particle size (x₉₉) of less than 5 μm,preferably of less than 3 μm, are necessary. FIG. 3 shows a sizedistribution of a pigment dispersion that is still suitable.

Preferably, according to WO 2011/026159 A1, the pigment dispersionshould contain between 10 and 50% of the flame-retardant substance.

Most prior art documents do not describe the influence of the dispersingagent in such detail as would be appropriate. Many chemicals, eventhough they provide an excellently stabilized flame retardantdispersion, have a negative impact on the spinning process because they,while also causing a modifying effect in the viscose thread, do notpositively influence the fiber strength as opposed to the modifiersused. Dispersing agents that have been found to be ideal for the flameretardant dispersion used to produce the inventive fibers and do notadversely influence the fiber strength are especially those that wereselected from the group comprising modified polycarboxylates,water-soluble polyesters, alkyl ether phosphates, end-group-capped nonylphenol ethoxylates, castor oil alkoxyl esters, and carboxymethylatedalcohol polyglycol ethers. Preferably, the pigment dispersion shouldcontain between 1.5 and 13% of the dispersing agent.

WO 2011/026159 A1 relates expressly to the production of staple fibers.Since they usually have a cutting length between approx. 25 and 90 mmand are mixed thoroughly several times prior to their final use intextiles, minor differences in the uniformity of the individual fibersas well as small spinning faults do not play a significant role in theirproduction. The situation is entirely different in the production of(continuous) filaments for textile or technical applications. They areusually spun out in thin filament bundles of approx. 10 to 2000individual filaments and reeled up directly. It takes approx. 48 hoursuntil a reel is full. Even if due to a spinning fault only oneindividual filament ruptures during these 48 hours, this will have asignificant impact on product quality and thus on the obtainable price.In addition, the filaments are naturally no longer mixed during theiruse so that any irregularities of the filaments become clearly visible,for example in textiles. This also affects the product quality and thusthe obtainable price.

Summing up, regenerated cellulose filaments that have high strength butexcessively coarse titers are known in the state of the art, as aretextile filaments that, even though they are so fine that they offer atextile wearing comfort, provide only low strength. In particular, notextile regenerated cellulose filaments are known that, while featuringhigh strength, also contain such a high proportion of pigment such asflame-retardant pigment that they actually exhibit good flame-retardantproperties.

SUMMARY OF THE INVENTION

Surprisingly, it was possible to solve this problem by using regeneratedcellulose filaments that have a pigment content of more than 20 wt % aswell as a strength (conditioned) greater than 22 cN/tex.

These pigments are used to incorporate desired additionalfunctionalities into the filaments. This may be, for example, flameretardation, a durable coloration, or a matting effect. Yet, for specialproducts, other additives can be used that, for example, provide forextremely good visibility or an extremely good warning effect,electrical conductivity, the absorption of pollutants, or radiographicvisibility (e.g. for surgical suture yarns). Generally, all such solidsare suited for use as pigments that, under the conditions in the viscosespinning solution, i.e., strongly alkaline and CS₂-containing, and inthe strongly acidic precipitation bath, will not undergo any undesiredchanges, especially that they will not go into solution, either partlyor entirely. Substances that, in a strongly alkaline environment, arepresent in a dissolved form and that, only in the acidic precipitationbath, pass into the form of a solid aggregate constitute a conceivableexception. In particular, in this case, the pigment is selected from thegroup comprising flame-retardant, colored, fluorescent (“high-vis”colorants), and radiographically detectable pigments. The presentinvention shall also expressly encompass mixtures of these pigments fora combination of several properties in the same filament, for examplethe combination of color pigments, high-vis color pigments, andflame-retardant pigments for use in light-fast, warning-colored,flame-retardant clothing for fire brigades and rescue services.

The described advantageous mechanical properties of the inventivepigment-containing regenerated cellulose filaments are achieved withparticular reliability if the pigment has a particle size distributionwith x₅₀ at less than 1.0 lam and x₉₉ at less than 5.0 μm, preferably atless than 3.0 μm.

The object resulting from the state of the art can be achievedparticularly well and in a surprising manner if the inventiveregenerated cellulose filaments have a fine single-fiber titer ofbetween 0.4 and 4 dtex, preferably of between 0.8 and 3.0 dtex. So far,it had not been possible to produce such fine regenerated cellulosefilaments with the above described strengths, especially if theycontained sufficient quantities of pigment.

Preferably, the spun-in pigments have a particle size distribution withx_(so) at less than 1.0 μm and x₉₉ at less than 5.0 μm, preferably atless than 3.0 μm.

The preferred organophosphorus compound used is2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide (FormulaI). This substance is available in sufficient quantities, for example,under the trade names Exolit and Sandoflam, and it is not washed out ofthe fibers during the manufacturing process and also during theirsubsequent use:

In a preferred embodiment, the inventive filament contains at least2.8%, preferably between 3.2% and 6.0%, more preferably between 3.5% and6.0% phosphorus, in each case related to cellulose. Lower phosphoruscontents than 2.8% do not yield a sufficient flame-retardant effect.Higher phosphorus contents than 6% cause the mechanical properties ofthe filaments to deteriorate and are also no longer cost-effective.

Most prior art documents do not describe the influence of the dispersingagent in such detail as would be appropriate. Many chemicals, eventhough they provide an excellently stabilized flame retardantdispersion, have a negative impact on the spinning process because they,while also causing a modifying effect in the viscose thread, do notpositively influence the fiber strength as opposed to the modifiersused. Dispersing agents that have been found to be ideal for the flameretardant dispersion used to produce the inventive regenerated cellulosefilaments that do not adversely influence the fiber strength areespecially those that were selected from the group comprising modifiedpolycarboxylates, water-soluble polyesters, alkyl ether phosphates,end-group-capped nonyl phenol ethoxylates, castor oil alkoxyl esters,and carboxymethylated alcohol polyglycol ethers. Preferably, the pigmentdispersion should contain between 1.5 and 13% of the dispersing agent.

Another object of the present invention is a non-pigmented regeneratedcellulose filament that has a strength in the conditioned state ofgreater than 36 cN/tex. Preferably, also the inventive regeneratedcellulose filaments have a fine single-fiber titer of between 0.4 and 4dtex, more preferably of between 0.8 and 3.0 dtex.

A regenerated cellulose filament having such high strength and finenessis well suited for processing into woven fabrics and other textilefabrics that—compared to standard viscose filament which has a strengthof approx. 20 cN/tex—have extraordinarily high abrasion resistance.Hence, it is particularly well suited for use in sports, for example formotorcycle garments, karting garments, and sportswear. In this context,abrasion resistance particularly refers to the property measured in theMartindale abrasion test. Due to the greater temperature resistance ascompared to synthetic filaments such as polyester and nylon, thehigh-strength filament is also used in many technical applications,e.g., for turbocharger hoses in passenger cars and trucks.

For the purposes of the present invention, both (non-pigmented)filaments having a strength in the conditioned state of greater than 36cN/tex and pigment-containing filaments having a pigment content of morethan 20 wt % and a strength (conditioned) of more than 22 cN/tex shallbe referred to as “high-strength”.

When being used, the inventive filaments are not used one by one, but inthe form of the filament bundles—also referred to as filament yarn—thatare obtained in the spinning process from each of the spinnerets. In thetextile sector, a filament yarn usually contains about 30-200 individualfilaments. In technical applications, for example for reinforcing cartires, conveyor belts, or other rubber goods, yarns usually containapprox. 700 to 2000 individual filaments.

Another object of the present invention is the use of the inventivefilaments for the production of a textile fabric. In addition to theinventive fibers, this fabric may also contain other fiber yarns orfilament yarns, for example and in particular, wool, flame-retardantwool, para-aramides (Kevlar®, Twaron®) and meta-aramides (Nomex®),polybenzimidazole (PBI), p-phenyl-2,6-bezobisoxazole (PBO), polyimide(P84®), Polyamidimide (Kermel®), modacrylics, polyamides,flame-retardant polyamides, flame-retardant acrylic fibers, melaminefibers, polyesters, flame-retardant polyesters, polyphenylene sulfide(PPS), polytetrafluoroethylene (PTFE), glass fibers, cotton, silk,carbon fibers, oxidized, thermally stabilized polyacrylonitrile fibers(PANOX®), elastanes, and electrically conductive fibers, as well asmixtures of these fibers.

Preferably, the fabric is a woven fabric, a knitted fabric, or acrocheted fabric. In the case of a woven or knitted fabric, mixing ofthe inventive filaments with the further fiber or filament yarns ispossible either by mixing prior to the production of the yarn, so-calledintimate blending, or by the combined use of pure yarns of the variousfilament and fiber types for weaving, knitting, or crocheting.

The inventive regenerated cellulosic filaments can be produced using aviscose process modified according to the invention. Therefore, it isalso an object of the present invention to provide a continuous methodfor the production of a high-strength regenerated cellulose filament byspinning of a viscose containing 4 to 8% cellulose, 5 to 10% NaOH, 36 to42% (related to cellulose) carbon disulfide, as well as 1 to 5% (relatedto cellulose) of a modifier into a precipitation bath, withdrawing thecoagulated filaments, a viscose being used whose spinning gamma value is50 to 68 and whose spinning viscosity is 50 to 150 falling ball seconds;and that the temperature of the precipitation bath is 34 to 65° C.,

Aa. the alkali ratio (=cellulose concentration/alkali content) of theready-to-spin viscose being 0.7 to 1.5;

b. the following precipitation bath concentrations being used:

-   -   H₂SO₄ 68-95 g/1    -   Na₂SO₄ 90-160 g/1    -   ZnSO₄ 30-65 g/1

c. the final withdrawal from the precipitation bath and reeling uptaking place at a velocity between 15 and 180 m/min, wherein

d. the filaments coagulated in the precipitation bath are subsequentlypassed through a second bath, the second bath containing aqueoussulfuric acid, 3-7 wt %, at a temperature of 80-98° C.

By spinning such a spinning solution followed by stretching and fixingin the second bath, one obtains high-strength regenerated cellulosicfilaments that, even after the incorporation of flame-retardant orcoloring pigments, have filament and/or yarn strengths that are markedlyabove those of comparable regenerated cellulosic filaments spun from atextile viscose spinning solution according to the state of the art.

Conveniently, a viscose is used to which the modifier has been addedonly shortly before the viscose is spun.

In the context of the inventive method, “continuous” shall mean thatspinning the viscose into the precipitation bath, stretching, washing,drying, and reeling up take place continuously in the same processingstep. This is contrasted by the widely used centrifugal spinning methodsin the course of which the moist filaments are wound up in centrifugesand these so-called spinning cakes are then washed and drieddiscontinuously.

Together, the measures proposed according to the invention, that is, tocomply with a certain spinning ripeness represented by the spinninggamma value, to comply with a certain viscosity represented by thefalling ball values, and to comply with certain conditions in theprecipitation bath, bring about the envisaged fiber properties. Thespinning gamma value denotes the proportion of carbon disulfidemolecules bonded to 100 cellulose molecules. The spinning gamma value isdetermined according to the Zellcheming Draft Leaflet by R. Stahn [1958]and to Leaflet III/F 2, respectively. The falling ball is used in thefalling ball method to determine viscosity; it is expressed in fallingball seconds. The determination is described in K. Götze, Chemiefasern[1951], p. 175.

In order to reliably achieve the high strength of the filaments, it hasproven advantageous to use pulp having an α-content of 93-99% ascellulosic raw material. Such pulp has only a small content oflow-molecular minor constituents that reduce the strength in thefinished filament, on the one hand, and may get into the precipitationbath circuits, on the other, where they may interfere in various ways ascontaminants.

An important feature of the inventive method is that the filaments arestretched by 70% to 105% in the second bath. In the state of the art, nosecond bath is used that contains only aqueous sulfuric acid and has atemperature of 80-98° C., on the one hand, while, on the other hand,upon exit from the precipitation bath, stretching is carried out by onlyapprox. 5% in the Continue spinning method and by approx. 10-20% indiscontinuous methods for textile filaments.

This method renders it possible to produce non-pigmented high-strengthregenerated cellulose filaments as well as such regenerated cellulosefilaments that have a pigment content of more than 20 wt % and astrength (conditioned) of more than 22 cN/tex.

In this context, a surprising connection was discovered between thestrength of a non-pigmented regenerated cellulose filament and that of apigment-containing regenerated cellulose filament if only the pigmentcontent in the filament is changed and the other process parameters aremaintained constant:

FFK _(P) =FFK _(R) ×c _(Cell) ²

wherein FFK_(P) is the fineness-related strength of thepigment-containing regenerated cellulose filament, FFK_(R) thefineness-related strength of the non-pigmented regenerated cellulosefilament, and c_(Cell) the cellulose content of the pigment-containingregenerated cellulose filament, related to the dry content of thefilament and expressed as a fraction.

For example, in inventive process conditions, in which non-pigmentedregenerated cellulose filaments having a strength of 35 cN/tex can beproduced, one also obtains pigment-containing regenerated cellulosefilaments having a strength of 22 cN/tex and a pigment content of 0.21(i.e., a cellulose content of 0.79).

Preferably, the pigments are spun in in the form of a pigmentdispersion. More preferably, the pigment dosing ratio is controlledand/or adapted automatically based on the spinning solution flow rateand is adjusted by means of a controlled dosing pump. Accurate dosing isextremely important, for example, in order to achieve a consistentfilament quality. In the textile sector, any deviation from suchconsistency will, in the final application, be clearly visible on thefabric. In technical applications, irregularities may cause failure ofthe final product.

Therefore, the selection of a suitable controlled dosing pump is ofimportance for the success of the inventive method. Eccentric screwpumps, for example, have proven to be particularly well suited. Pistonpumps, in turn, are typically used for large streams of liquid, however,they are not accurate enough for the small streams of liquid encounteredin this case, as they do not function continuously, but in pulses,thereby causing a constantly changing pigment concentration. The gearpumps widely used in the synthetic fiber sector are not suitable for usein the present method, either, because, on the one hand, the pigmentsused, especially TiO₂, would lead to very high abrasion on the gearpumps. On the other hand, there exists the risk of the pigment beingdeposited between the teeth of the pump gears, clogging them up so thatthe output is decreasing and the dispersion enters the viscose spinningsolution with a lower pigment content.

According to the invention, all such solids are generally suited for useas pigments that, under the conditions in the viscose spinning solution,i.e., strongly alkaline and CS₂-containing, will not undergo anyundesired changes. In particular, in this case, the pigment is selectedfrom the group comprising flame-retardant, colored, fluorescent(“high-vis” colorants) and radiographically detectable pigments. Thepresent invention shall expressly also encompass mixtures of thesepigments for a combination of several properties in the same filament.In a preferred embodiment of the invention the pigment-type substance istherefore at least partly a flame-retardant substance.

In order to achieve an effective flame-retardant action, in theinventive method such a quantity of the flame-retardant substance isspun in that the finished fiber contains at least 2.8%, preferablybetween 3.2% and 6.0%, more preferably between 3.5% and 6.0% phosphorus,related to cellulose. Preferably, phosphorus is present in the form ofthe organophosphorus compound2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide (I).

The described advantageous mechanical properties of the regeneratedcellulose filaments obtained according to the inventive method areachieved with particular reliability if the pigment dispersion containsbetween 10 and 50% of the flame-retardant substance with an averageparticle size (x₅₀) of less than 1.0 lam and a maximum particle size(x₉₉) of less than 5.0 μm, preferably smaller than 3.0 μm, as well asbetween 5 and 20% of a dispersing agent.

Preferably, this dispersing agent for the flame retardant dispersion isselected from the group comprising modified polycarboxylates,water-soluble polyesters, alkyl ether phosphates, end-group-capped nonylphenol ethoxylates, castor oil alkoxyl esters, and carboxymethylatedalcohol polyglycol ethers.

According to the invention, the flame-retardant phosphorus compound thatis produced as a pigment is added to the viscose spinning solution inthe form of a pigment dispersion. In this process, so much of theflame-retardant substance is spun in that the finished fiber contains atleast 2.6%, preferably between 3.2% and 6.0%, more preferably between3.5% and 6.0% phosphorus, related to cellulose.

As has already been mentioned hereinabove, a flame-retardantorganophosphorus compound that is particularly well suited for thepurposes of the present invention is2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide.

The quality of the pigment dispersion, in particular, also has asignificant influence on fiber properties. This quality depends on theaverage and maximum particle sizes of the pigments, on the concentrationof the dispersion in use, i.e., when adding it to the viscose spinningsolution, as well as on the type and the quantity of the dispersingagents.

Contrary to the possible upper particle size of 10 μm described in thepatent EP1882760, it has been found that an average particle size (x₅₀)of less than 1 μm and a maximum particle size (x₉₉) of less than 5.0 μm,preferably of less than 3 μm, are necessary. FIG. 3 shows a sizedistribution of a pigment dispersion that is still suitable.

Preferably, the pigment dispersion should contain between 10 and 50% ofthe flame-retardant substance.

The inventive method will now be described based on a feasible and wellsuited embodiment, without intending to limit the invention to suchembodiment. The invention expressly encompasses functionally equivalentsolutions, as well.

FIG. 1 shows in principle the spinning machine that is used to carry outthe method: The viscose or the viscose dispersion mixture is fed via thepipe line system 1. Dosing of the titer-dependent viscose quantity iscarried out by means of precision gear pumps 2. In the filters 3, thedosed viscose is filtered. In the variant described herein, typicalspinnerets 4 are installed as submerged in the precipitation bath 5 onso-called spinning pipes for the production of the required filament.Preferably, in a precipitation bath, two filament yarns from discretespinnerets 4 are spun side by side and are then guided parallel to oneanother across the entire spinning machine. These spinnerets 4 containthe number of holes that corresponds to the desired number of individualfilaments in one filament yarn. The precipitation bath 5 is suppliedwith precipitation bath liquid of the inventive composition via alevel-controlled inlet. Via a pipe line, the coagulated filaments,together with the precipitation bath liquid, first arrive at a trough 7that serves as drain for the precipitation bath. Such a method isusually also referred to as a tube spinning method. Above the trough 7,the filaments are guided onto the first godet 8 of the stretchingstation where excess precipitation bath is collected by the trough 7.From the godet 8, the filaments run on to the second bath 9 where theyare stretched and fixed at the same time. This second bath has an inlet10 and a drain 11 for the above described bath liquid. From thedeflection device arranged in the second bath, which may be a movable(not driven) pair of rollers or also a rigid glass rod or the like, thefilaments are guided to a interlocked pair of rollers equipped with awashing water feeder 12 and a washing water drain 13, where they arefirst washed and then dried in a drying zone 14. Subsequently, they arefinished in a finishing station 15 and finally reeled up on a twin reelstation 16. The pair of rollers can additionally be equipped with anaspirator 17 for the generated steam and other gaseous secondaryproducts.

The tube spinning method described herein achieves an approx. 10%greater strength as well as faster to approx. three times fasterspinning than the conventional immersion spinning method. Without theaddition of a pigment dispersion, spinning velocities of up to 180 m/minare possible, with the addition of a pigment dispersion, spinningvelocities of up to 85 m/min are still possible.

Particularly regular admixing of the pigments can for example beachieved with a dosing apparatus as shown in FIG. 2: It comprises anagitator vessel 18 used to prepare the pigment dispersion, a dispersiondosing pump 19 used to deliver the dispersion, an appropriate flowmeasurement 20 as well as two static mixers 21, 22 used to mix thedispersion with the main stream of the viscose spinning solution which,via a conduit 23, a pressure booster pump 24, and a flow measurement 25,arrives at the static mixers 21, 22. Following another flow measurement27, the viscose pigment dispersion is guided to the spinning machine viaanother pipe line 28. In addition, a temperature control circuit 27 canbe provided for controlling the temperature of the main viscose stream.

The invention will now be explained based on examples. These examplesshall be construed as possible embodiments of the invention. By no meansis the invention limited to the scope of these examples.

EXAMPLE 1

6 parts by weight of2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphoriane]2,2′disulfide, 6 partsby weight of water, and 0.55 parts by weight of alkyl polyglycol etherphosphoric acid ester are homogenized using a dissolver and ground in anagitator bead mill (Drain, type Perl Mill PML-V/H) with zirconium oxidegrinding beads at a temperature of 40-55° C. until the finisheddispersion has an x₉₉<1.50 μm. Beech pulp (R18=97.5%) was alkalized withmashing lye that contained 240 g/1 of NaOH at 35° C., while beingstirred, and pressed into an alkali cellulose nonwoven. The alkalicellulose nonwoven was defibered, ripened, and sulfidized. Using dilutedcaustic soda, the xanthogenate was dissolved into a viscose with 5.6%cellulose, 6.8% NaOH, and 39% CS₂, related to cellulose. The viscose wasfiltered 4 times and vented. 1 hour prior to spinning, 3% (related tocellulose) of ethoxylated amine, a modifier causing a sheath structure,was added to the viscose. The viscose was post-ripened to a spinninggamma value of 57. The viscosity during spinning was 80 falling ballseconds. The finished flame retardant dispersion is added to thisready-to-spin viscose.

The spinnerets used have a spinneret hole diameter of 60 μm. Theprecipitation bath contains 75 g/1 of sulfuric acid, 113 g/1 of sodiumsulfate, and 53 g/1 of zinc sulfate. The precipitation bath temperaturewas 39° C.

The coagulated and partly regenerated plastic thread strand of a paleyellow color was guided via a godet G1 into a second bath whosetemperature was 95° C. and that contained 4.8 wt % of sulfuric acid,where it was stretched by 100% between G1 and a second godet G2. Thefinal withdrawal was carried out at a velocity of 30 m/min. The filamentwas then washed acid-free with hot water, dried, and subsequently reeledup.

TABLE 1 Fiber data BISFA Individual wet Titer filaments FFk FDk modulusP content Fiber [dtex] [number] [cN/tex] [%] [cN/tex] [%] Example 1 20076 23.5 6.1 5.2 3.5

1. A regenerated cellulose filament, wherein the filament has a pigmentcontent of more than 20 wt % and a strength in the conditioned state ofmore than 22 cN/tex.
 2. The regenerated cellulose filament according toclaim 1, wherein the pigment is selected from the group consisting offlame-retardant, colored, fluorescent and radiographically detectablepigments.
 3. The regenerated cellulose filament according to claim 2,wherein the pigment has a particle size distribution with x₅₀ at lessthan 1.0 μm and x₉₉ at less than 5.0 μm.
 4. The regenerated cellulosefilament according to claim 2, wherein the filament has a single-fibertiter of between 0.4 and 4.0 dtex.
 5. The regenerated cellulose filamentaccording to claim 2, which comprises at least 2.8%, phosphorus relatedto cellulose.
 6. The regenerated cellulose filament according to claim2, wherein the phosphorus is2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide (I). 7.The regenerated cellulose filament according to claim 2, wherein thefilament comprises a dispersing agent selected from the group consistingof modified polycarboxylates, water-soluble polyesters, alkyl etherphosphates, end-group-capped nonyl phenol ethoxylates, castor oilalkoxyl esters, and carboxymethylated alcohol polyglycol ethers.
 8. Aregenerated cellulose filament having a strength in the conditionedstate of greater than 36 cN/tex.
 9. A textile fabric comprising theregenerated cellulose filament according to claims 1 or
 2. 10. Acontinuous method for the production of a high-strength regeneratedcellulose filament comprising: 1) spinning a viscose containing 4 to 8%cellulose, 5 to 10% NaOH, 36 to 42% (related to cellulose) carbondisulfide and 1 to 5% (related to cellulose) of a modifier into aprecipitation bath to form coagulated filaments, 2) withdrawing andreeling up the coagulated filaments, wherein: a viscose is used whosespinning gamma value is 50 to 68 and whose spinning viscosity is 50 to150 falling ball seconds; the temperature of the precipitation bath is34 to 65° C.; the alkali ratio (=cellulose concentration/alkali content)of the ready-to-spin viscose is 0.7 to 1.5; the bath concentrations are:H₂SO₄ 68-95 g/1 Na₂SO₄ 90-160 g/1 ZnSO₄ 30-65 g/1; and the withdrawingand reeling up take place at a velocity between 15 and 180 m/min; and 3)passing the coagulated filaments in the precipitation bath through asecond bath, wherein the second bath comprises aqueous sulfuric acid,3-7 wt %, at a temperature of 80-98° C. to form the regeneratedcellulosic filaments.
 11. The method according to claim 10, wherein pulphaving an α-content of 93-99% is used as cellulosic raw material. 12.The method as according to claim 10, wherein the coagulated filamentsare stretched by 70% to 105% in the second bath.
 13. The methodaccording to claim 10, wherein a pigment-type substance in the form of apigment dispersion is spun into the filaments.
 14. The method accordingto claim 13, wherein the pigment dosing ratio is controlled and/oradapted automatically based on the spinning solution flow rate andadjusted by means of a controlled dosing pump.
 15. The method accordingto claim 14, wherein the controlled dosing pump is an eccentric screwpump.
 16. The method according to claim 13, wherein the pigment-typesubstance is selected from the group consisting of flame-retardant,colored, fluorescent, and radiographically detectable pigments.
 17. Themethod according to claim 16, wherein the pigment-type substance is atleast partly a flame-retardant substance.
 18. The method according toclaim 17, wherein the high-strength regenerated cellulose filamentcomprises at least 2.8% phosphorus related to cellulose.
 19. The methodaccording to claim 18, wherein the phosphorus is2,2′-oxybis[5,5-dimethyl-1,3,2-dioxaphosphorinane]2,2′disulfide (I). 20.The method according to claim 17, wherein the pigment dispersioncontains between 10 and 50% of the flame-retardant substance with anaverage particle size (x₅₀) of less than 1.0 μm and a maximum particlesize (x₉₉) of less than 5.0 μm and between 1.5 and 20% of a dispersingagent.
 21. The method according to claim 17, wherein a dispersing agentis used to disperse the flame retardant substance, wherein thedispersing agent is selected from the group consisting of modifiedpolycarboxylates, water-soluble polyesters, alkyl ether phosphates,end-group-capped nonyl phenol ethoxylates, castor oil alkoxyl esters,and carboxymethylated alcohol polyglycol ethers.
 22. The methodaccording to claim 10, wherein the spinning of the viscose into theprecipitation path takes place in the form of a tube spinning method.23. The flame-retardant regenerated cellulose filament according toclaim 5, which comprises between 3.2% and 6.0% phosphorus related tocellulose.
 24. The flame-retardant regenerated cellulose filamentaccording to claim 5, which comprises between 3.5% and 6.0% phosphorusrelated to cellulose.
 25. The method according to claim 17, wherein thehigh-strength regenerated cellulose filament comprises between 3.2% and6.0% phosphorus related to cellulose.
 26. The method according to claim17, wherein the high-strength regenerated cellulose filament comprisesbetween 3.5% and 6.0% phosphorus related to cellulose.